Secret signaling



J 1927' R. E. coRAM SECRET SIGNALING Filed Nov. 21, 1923 LOW PASS

6 telephonic or Patented June 14, 1927.

UNITED STATES PATENT not: a. com, or nnwaamnnw JERSEY, .Assrenoa 'rownsrnmt ELECTRIC com- PANY, INCORPORATED, or new 20111:, n. Y., Aconronarron or new YORK.

srionnr SIGNALING.

Application filed November 21, 1923. Serial No. 676,081.

to render it incapable of intelligible recep-- tion by ordinary means.

It has been proposed to obtain secrecy 1n other messages by confusingthe message at the transmitting sta t1on and automatically recasting oruntanglmg it at the receiving station, thus rendering 1t practicallyimpossible for an unauthorized person to obtain the informationcontained therein. It has been proposed to accomplish this by modulatinghigh frequency carrier waves by waves within the speech frequency rangein a thermionic modulator, then suppressmg the original carrier and theupper side band and transmitting only the lower side band. This lowerside band represents an inverse frequency speech wave which may berecast into intelligible form at the receiving station by reversing theprocess by which the signal has been confused. It has also been proposedto vary the frequency of the carrier wave'continuously and in a predetermined order to lessen the possibility of; its intelligible reception byan outsider. In such a system, it is important to suppress theunmodulated carrier wave component to prevent the carrier frequency, andthe scheme of varying it, from becoming known, and it is likewisenecessary to suppress one side band of the modulated carrier wave,since. if both side bands were transmitted, a ,Simple detector wouldyield currents of the double frequencies of the voice which are to '40 acertain degree intelligible.

In systems of the character described, the unmodulated carrier wave andits upper side band have been eliminated quite successfully by means of'filters, but another and more formidable difliculty is presented whichit is difiicult to overcome by ordinary means, namely, since thethermionic modulator acts to a certain extent as an amplifien'theoriginal currents of signal or speech frequencies,

are not entirely suppressed in the modulator and are partly passed tothe line. If these currents are allowed to be transmitted, even 1n verysmall amounts, it mlght be possible to separate them by filters and toamplify them to the desired degree, thus defeating the pur ose of thesystem. In order to substantial y suppress currents of speechfrequencies in a thermionic modulator, a complicated and expensivesystem is required. This difficulty is entirely overcome by the presentinvention.

, In accordance with a feature of the in vention, a high frequencycarrier wave is modulated by waves of speech frequency in such a manneras to completely suppress the original speech, currents, the unmodulatedcarrier wave-and its upper side band then being eliminated and only thelower side band passed to the line, More specifically, the speechcurrents are completely suppressed in a simple and inexpensiveelectromagnetic modulator, and the unmodulated carrier wave and itsupper side band are eliminated by filtering and the lower side bandalone is transmitted to the receiving station without distortion.

Other features and advantages of the invention will appear from theconsideration of the following description taken. in connection with theaccompanying drawing, in

magnetic telephone repeater which may be of the type disclosed in PatentNo. 1,156,636, issued October 12, 1915 to H. E. Shreeve, the incomingvoice currents actuating the repeater in the usual manner. The carrierwaves are impressed on a circuit 5 including the transmitter microphone4 of the electromagnetic modulator M by means of ahigh frequency carriersource 6 coupled thereto by means of a transformer 7 The varyingresistance of the transmitter button of the microphone 4: varies theamplitude of the carrier current which is circulated through it. Thisvariation in amplitude results in three frequencies being impressed uponthe circuit 5, namely, the original carrier, the

carrier plus the speech currents and the carrier minus the speechcurrents. The original rents of higher frequencies.

speech currents are completely suppressed in the modulator, no part ofthem being passed to the line. Thextransformer 8 connects the low passfilter-9 may be designed in ac-' cordance with the principles set forthin Patent -No. 1,227,113 issued May 22, 1917 to G. A. Gampbe1l,'and' isso arranged as to, pass with approximately negligible attenuationcurrents'of frequencies lower than a fixed cut-off frequency, as forexample, 3,000 cycles, and substantially extinguishes cur- If,therefore, the current generated by the high frequency source 6 isapproximately 3,300 cycles, it will be evident that this unmodulatedcarrier current as well as the side band comprising the sum-component ofthe original carrier and the low frequency speech currents will besuppressed by the low pass filter 9, and. the filter will pass only theside band comprising the diflerence component of the original carrierand the speech waves. Since the resultant wave passed'by the filter 9 isof a frequency equal to the original carrier minus the input speech, itis an inverse function of the speech currents, or in other words, aninverted wave, and is therefore incapable of intelligible "reception byordinary means. In practice, the inverse speech currents are-transmittedover a telephone circuit to a receiving station, not shown, or they maybe used to modulate a radio -or carrier wave for transmission to thedistant station -where they are recast.

into intelligible form by means of a suitable demodulator. This may beaccomplished by passing the incoming waves at the receiving stationthrough a duplicate of the modulat ing system shown, the incominglineterminating in the receiving coil of the repeater, to restore thespeech frequencies to their normal order.

A modification of the invention is shown in-Fig. 2 in which a push-pullmodulator circuit is used with a-.carrier frequency which, for ordinarypurposes, may be approximately 3,000 cycles. The out ut of the lowfrequency circuit 2 is coupled y means of a transformer 10 to afilt'rlng circuit including the low pass filter- 11' so designed as totransmit freely currents of the frequencies from 'zero to an upper limitof preferably about 3,000 cycles, or at least sutficientlyhigh inconnection with Fig. 1 except that the microphone element 4 is of thepush-pull type having two electrodes which are connected respectively tothe ends of the-primary of transformer 8 and a diaphragm connectedthrough the secondary of transformer.

7 'to the midpoint of the primary of transformer 8. The carrler' wavesgenerated by the source 6 are thus impressed upon two balanced parallelcircuits each of which include one electrode and the diaphragm of themicrophone element 4., and the unmodulated carrier current is balancedout so that the current induced in the secondary of .transformer 8consists of only two frequencies, namely, the carrier plus the speechcurrents and the carrier minus the speech currents. This modulatingcircuit thus pre vents the transmission of substantially all unmodulatedcurrents, since the original speech currents are completely suppressed1n the electromagnetic repeater,.and the unmodulated carrier is balancedout in the balancedcircuits of the modulator. The low pass filter9,which is connected to, thesecondary of transformer 8, suppresses theupper side band and permits the transmission of only the lower sideband. This inverted frequencies. In order to restore the speechfrequencies to their normal order, it is necessary to 'passthe invertedspeech through a duplicate of the modulating apparatus as in the case ofthe system described in connection with Fig. 1. n

In Fig.3, amodification of the transmission system described inconnection with Fig. 2 shown. In this system, two electromagnetlctelephone repeaters are employed having their receiving coils 3 and 3coupled to the speech frequency line 2 by means of a transformer 12having two secondary windings. .The electrodes 4 and 4 of the repeaterare connected'respectively to, the ends of the primary of transformer 8and their dia-" phragms are connected through the secondary oftransformer 7 to the midpoint of the primary of transformer 8. Thevarying resistance of the transmitter electrodes 4 and I 4? oftheserepeaters varies the amplitude of the carrier current generated bythesource 6, this system operating in a manner similar to that shown inFig. 2, the two repeaters taking the place of a single repeater having apush-pull transmitter. The original speech currents are completelysuppressed in the electromagnetic repeater and the unmodulated carriercurrent is balanced out in the parallel circuits described above so thatno direct speech or unmodulated carrier waves are passed to the km. 'Theupper" unmodulahd carrier and one of its side bands. It may also benoted that in this system any degree of modulation may be obtaineddepending on the constants of the electromagnetic modulators employed.

It will be understood that the invention is not to be construed aslimited to the actual frequency values that have been given above by wayof illustration, but that frequencies higher or lower than those aboveassigned to source 6 may be used. So long as the resulting lower orinverse side band lies in the audio-frequency range it will be heard inan ordinary receiving circuit as noise, and will tend to confuse or maskthe signal.

Although the invention has been shown and described in connection withcarrier secrecy systems, it will be understood that it is equallyapplicable to radio signaling systems. Furthermore the invention is notto be limited either in these or other respects by the specificembodiments and details herein shown and described, except as defined bythe scope of the appended claims.

What is claimed is: i

1. In combination, a circuit including a source of sustained carrierwaves of a fre quency at least as high as the upper essential speechfrequencies, a mechanically variable resistance controlled in-saidcircuit to modulate said carrier waves, said resistance being varied toproduce modulated carrier waves having a lower side band within theaudio frequency range, and a filter for tr-ansmit-.

ting only said lower side band.

2. In combination, a tI'aIISXIIISSIOD medium,

a circuit including a source of sustained carrier waves associatedtherewith, said waves having a frequency at least as high as the upperessentlal speech frequencies, mean for modulating said carrier waves inaccordance with speech'waves while preventing speech currents from beingset 'up in said circuit, said modulated waves having a lower side bandwithin the audio frequency range, and means for transmitting only thelower side band within the audio frequency range.

3. In combination, a modulator circuit comprising a source of sustainedcarrier waves of a frequency at least as high as the upper essentialspeech frequencies, a mechanically variable resistance in said modulatorcircuit adapted to be controlled in accordance with speech waves tomodulate said carrier waves and to suppress speech currents fromtransmission to said modulator circuit, and an outgoing circuit having afilter included therein for limiting the transmitted frequencies toaudio frequencies lower than the frequency f said source of'carrierwaves.

4. A. secret signaling system comprising an electromagnetic repeaterhaving a receiving coil and a microphone element, means for circulatinga current of high frequency near the upper limit of the audio frequencyrange through said microphone clement, means for passing currents ofspeech frequency through said receiving coil to modulate said highfrequency current, and means for suppressing the unmodulated highfrequency current component. and the upper side band produced bymodulation and for transmitting the lower sideband Within theaudio-frequency range.

5. A secret signaling system comprising an electromagnetic repeaterhaving a receiver c011 and a microphone ele1nent, means for circulatinga current of high frequency near the upper limit of voice frequencyrange. through said microphone element,

means for passing currents of speech frequency through said receivingcoil to modulate such high frequency current without setting up speechcurrents in the circuit of the microphone element, and a filter adaptedto'suppress currents of the frequency of the unmodulated high frequencycurrent component and the upper side band produced by modulation and totransmit only modulated currents within the voice frequency range.

6. In a secret transmission system, a pushpull-electromagnetic repeatercomprising a receiving coil and a microphone element having twoelectrodes, means-for impressing a current of high frequency near theupper limit of the voice frequency range upon two balanced circuits eachincluding one of said electrodes, means for passing currents of speechfrequency through said receiving coil to modulate said high frequencycurrent without setting'up speech currents in the circuits of the twoelectrodes, and a lowpass filtel for suppressing the unmodulated highfrequency current and the upper side band.

7. In a secret transmission system, a pair of electromagnetic repeaterseach having a receiving coil and a microphone element, means forimpressing a current of high frequency near the upper limit of the voicefrequencv range upon two balanced circuits each including one of saidmicrophoneelements, said high frequency current havin a frequency nearthe upper limit of speech requencies, means for passing currents ofspeech frequency through the receiving coils of said repeaters tomodulate the high frequencycurrent and produce a lower sideband withinthe voice frequency range, and a low pass filter for suppressing theunmodulated high frequency current and the upper side band.

8. In a secret transmission system, a circuit, means for impressing acurrent of high frequency near the upper limit of the audio frequencyrange upon said circuit, a second circuit, means for impressing currentsof speech frequency upon said second circuit,

' an electromagnetic device in said second circuit controlled by saidspeech currents, means controlled by said device for modulating the highfrequency current in said first circuit in accordance with said speechcurrents and for preventing the transmission of said speech currents tosaid first circuit, a third circuit-coupled to said first circuit, andmeans in said third circuit for suppressing the unmodula-ted highfrequency current and the upper side band produced by modulation and fortransmitting modulated currents Within the audio frequency range.

In witness whereof, I hereunto subscribe my name this 19th day ofNovember A.-D.,

ROY E. CORAM.

